Diamond abrasive wheel



6, 9 L. H. MILLIGAN E TAL I 2,479,078

DIAMOND ABRAS IVE WHEEL Filed Oct. 20, 1945' FUZIJDZJE JYWMM LUWELL. H, 'MIL .I IGAN RUEERT H. LUMEARI! MMQTX Patented Aug. 16,1949

DIAMOND ABRASIVE WHEEL Lowell n. Milligan and Robert 11. Lombard, --Worcester, Mass., assignor's to Norton Company, Worcester, Mass., a corporation of Manachusetts Application October 20, 1945, Serial No. 623,578 19 Claims. (01. 51409) This invention relates to diamond grinding wheel construction and to a method-of achieving the same.

In the construction of grinding wheels utilizing costly abrasive materialsgsuch as diamond grain, diamond dust, bort, or the like, of the so-called cup type, it is frequently necessary to mount the annulus carrying the diamond abrasive upon a supporting disk or back that has to be of a material or composition different from that of the abrasive annulus itself. A number of factors complicate the making of such an assembly, not the least of which is differences in the thermal coeflicients of expansion of the respective materials or parts that have to be used. Various expedients have heretofore been suggested for making such an assembly; some are not workable, others are complicated, particularly as to procedure, and some have detrimental effects upon operation of the utimate wheel assembly. For example, it has heretofore been proposed to utilize a junction or joint made of a rubber or rubber-like material in such a manner that the joint is resilient, in the endeavor to provide compensation or some sort of self-readjustment with respect to the forces that come into action; but such a' resilient junction can have the detrimental action of permitting a shift of the abrasive annulus relative to its supporting disk or center, a shift, either axially or radially, instigated by the reaction upon the abrasive annulus of,the direction of the pressure exerted between it and the workpiece. As a result, the abrasive annulus is forced out of truth.

Similar considerations apply to diamond grinding wheels built up out of segments, andto other abrasive articles.

Such defects and disadvantages can be greatly aggravated by other factors such as the size and weight of the abrasive annulus and also the axial dimension of it, particularly where a substantial depth" of cup of the wheel is necessary. Among the objects of this invention is to provide a cup type of grinding wheel construction and method tively described herein, and the scope of the application of which will be indicated in the following claims. I

In the accompanying drawings, in which are shown by way of illustration several of the various possible embodiments of our invention,

Fig. l is a diametrical cross-section of a diamond abrasive cup-wheel of one form, and

Fig. 2 is a diametrical cross-sectional view of a diamond abrasive cup-wheel of another form.

Referring first to Fig. 1, let it be assumed that a disk-like back or center 13 has to have assembled to it an abrasive annulus A containing costly abrasives such as diamond grain so that therefore the depth or axial dimension of the abrasive annulus A is necessarily very small; thus it may be on the order of or of an inch. In such case the annulus A is preferably of composite or builtup and unitary construction, as later described. Let it also be assumed that for other reasons it is desired that the diamond grain be bonded by a ceramic or vitrified bond. The back B may be made of a material such as steel or other metal, cured resins, usually synthetic, such as phenolic formaldehyde resin, aniline formaldehyde resin, or other so-called reslnoids which may include hardened, cured, or vulcanized natural or synthetic rubber or vitrified ceramic material; or it may comprise resinoid-bonded metal particles or powder of any suitable character such as, for example, powdered aluminum. In many instances a resinoid back is preferable over a solid metal back, particularly in large sizes of wheels where the factor of weight of the metal back becomes a disadvantage.

Such materials for the back B are likely to have thermal coemcients of expansion different from that of the abrasive annulus A and in some cases the divergence or difference is very substantial. Moreover, where the abrasive annulus A has its diamond grain bonded by a ceramic or vitrified or like glassy bond, the diamond grains must be held tightly so that their full capacity for abrasive action, namely, to grind for a long time without appreciable dulling, may be fully made use of; hence it is desirable that the ceramic or vitreous bond diamond-containing body be strong and relatively dense or of relatively low porosity, resulting in an abrasive structure that is of socalled hard grade, having a modulus of elasticity that is relatively high. A weak bond structure or so-called soft-grade abrasive structure, usually of high porosity, would permit the diamond grain to be lost by breaking away from the bond structure before the useful life of the diamond grains 3 has been utilized. The hard-grade vitrifiedbonded abrasive annulus A, having a relatively high modulus of elasticity, is, for the above reasons, relatively stifi and resists bending or distortion. If directly joined to the back B, the thermal coefficient of expansion of which is quite different as above pointed out, itis possible to overstrain the abrasive annulus A and fracture it or to overstrain the cementing material that joins it to the back, and thus cause fracture by shear or otherwise. It is such difficulties and obstacles as have just been noted that we are enabled to overcome according to our invention.

To illustrate certain features of our invention, where it is desired or necessary to produce a cupwheel of substantial axial dimension or depth, as indicated in Fig. 1; and such a depth illustratively may be on the order of four or five inches with a mean diameter on the order of fourteen inches, we make up a suitable number of ring elements, illustratively four in number and indicated in Fig. 1 at R R R and R out of mixes that include vitrifiable or ceramic bond material, selecting ingradients or proportions of ingredients, or controlling porosity, or any combination of these factors, so that the rings will have thermal coefficients of expansion that are graded progressively between that of the diamond abrasive annulus and that of the back B; with such an arrangement of the parts superimposed upon each other as in Fig. 2, in the proper order according to their thermal coeflicients, we can rigidly secure successive parts together by suitable cementing materials or bonds which do not have to be resilient and which upon being set can have a relatively high modulus of elasticity. This is of substantial-advantage in manufacture because relatively simple procedures may be employed and cementing or bonding materials used that are readily available or may be readily prepared. For example, the rigid junctions may be made out of cements having synthetic resins, such as phenol formaldehyde, aniline formaldehyde, etc., as a base, being applicable in liquid or semi-liquid condition. Some of them are heat-curable at convenient temperatures; others are cold-setting in that they set or cure at ordinary room temperatures. including compounds made up of natural rubber or any of the synthetic rubbers, preferably compounded and cured or vulcanized to relatively hard condition, inasmuch as a characteristic of resiliency is preferably avoided where it is desired not to run the risk of reactions to grinding pressures forcing the annulus out of truth. Or we may employ at all of the junctions between ceramic parts, including the junction between the bottommost ring element R and the back B when the latter is of ceramic character, a junction effected by firing so as to flow ceramic or vitrifiable bond material together from contiguous faces of the superimposed parts; or we may also utilize and apply an appropriate junction layer of vitrifiable or ceramic bond material which fuses during firing and upon cooling off forms a strong glassy layer-like vitrified or ceramic joint united with the structures of adjacent ceramic parts and with bond material of the latter; if,

however, the back B is of non-ceramic character,

the junction between it and the bottommost ring element is effected by any of the above-mentioned cements.

As is later pointed out, where it is desired to achieve certain other advantages and results, particularly with respect to improving and sup- Or we may employ other resinoids,

plementing strain-resisting characteristics of the annular abrasive portion G which, as later described, is relatively thin and contains preferably diamond abrasive grains, the above-described 5 junctions are preferably made of layers of vitrified or ceramic bond material rather than of cements such as the organic cements illustratively described; though the latter serve satisfactorily for many purposes of our invention, they have, even if cured to hard condition, a property of some yieldability, analogous to ductility, when subjected to localized or concentrated forces or pressures and hence, under such concentrated stress, they partake of strain and hence some deformation. This is not true where the junctions are effected by the glassy layer-like vitrified or ceramic materials above-described and the latter therefore make possible the achievement of other advantages and results later described.

With such a construction as is above described and employing joining or junction materials as illustratively above set forth, detrimental effects or stresses or strains caused by changes in temperature of the wheel structure do not take place and what stresses do occur are well confined within the capacity of the particular part to take a corresponding strain, and the factor of safety can thus be materially increased. The temperature changes that the wheel structure undergoes in practice are, first, from ordinary room temperatures up to operating temperature, usually limited or controlled with the aid of a suitable coolant liquid; when the grinding operation is ended, and this may run for long stretches of time, the wheel structure partakes of a second change in temperature, namely, from operating temperature down. to room. temperature. However, substantial variations in temperature during grinding operation may take place. For example, the grinding operation may have to be a dry" grinding operation in which substantial elevation in temperature of the wheel structure can take place; or, during a continued grinding operation, the grinding pressure and hence the depth of cut taken by the wheel, changed from time to time; or the kind of coolant liquid or its rate of application changed, according to circumstances. Also, such changes in temperature may be accompanied by variation in rate of tem- 50 perature change,

Though the relative dimensional change of the abrasive annulus A and the back B, for such temperature changes, may be relatively large or substantial, because of a corresponding difference in 55 the thermal coeflicients of expansion of the two parts, each of the parts according to my invention is prevented from subjecting the other or an intermediate cementing material to the substantial or large stress or force which the relative dimen- 60 sional changes in the two parts could produce, and such stress is broken up into a succession of increments, individually harmless, for action upon a corresponding number of interposed ring elements which because of their graded or 65 progressive thermal coefficients as above described go along with these respective increments of overall stress and each ring element is at no time over-burdened by strain imposed upon it by the adjacent ring or other elements to which 70 it is rigidly joined.

Thus, referring to Fig. 1, let it be assumed that the structure is at room temperature and undergoes heating to its operating temperature. For each increment of temperature rise, each of the 75 elements B, R, R, R", R and A expands, in-

the ring element R 5 creasing in diameter, each at a progressively lesser amount. in the order just named, where the thermal coefficient of expansion of the back 3 is greater than that of the abrasive ring A. For each increment of temperature rise, the relatively large increase in diameter of the back B is accompanied by a slightly lesser increase in diameter of and hence with a strain, usually in shear, imposed upon the Junction J between these two parts that is well within the capacity of the material of that Junction even if it is of high modulus of elasticity; relative to each other, the back B can be in compression and the ring element R in tension, due to such relatively small stress as the back B, in expanding at a slightly greater rate than does the ring element It, impose upon the latter. Blit'as that is taking place, the ring element R is expanding, but its increase in diameter for the given increment of temperature increase is less than the increase in diameter of the ring element R. Relative to each other, ring element R is in compression and ring element R. is in tension to the extent of the stress transmitted from element R. to element B through the junction J therebetween. Such actions as these, for each increment oi temperature rise, take place between successively adjacent ring elements and the interposed junctions or joints. Thus, ring elements I R and R coact in a similar manner with each other and uponthe intervening junction .1; then ring elements R and R coact in a similar way with each other and the interposed Junction J and so on, ring element R and the abrasive element A in the illustration of Fig. i being the last two so to coact with each other and with the interposed junction .7.

, In this manner no single element is overstrained or overstressed by its adjacent one; each element absorbs or takes only a fraction of the otherwise substantial total or aggregate strain or stress differential that could otherwise exist between the parts 13 and A because of their diver- 6 priate or desired thickness, the two portions G and U beingi preferably molded successively, under pressure, in the same mold, with the layer U, during molding, superimposed upon the abrasive layer G. The portion U may be compounded'to include any suitable ingredients to give it when fired the desired hardness to back up the relatively thin abrasive portion G against the grinding pressures and if desired a thin layer of raw bond material may be interposed between the two during molding to form when flred a hard and resistant direct under-support for the abrasive portion G as well as to form a stronger union or Junction between it and the underportion U. In

so constructing the abrasive annulus A whereit contains diamond grains or the like, it is easier and less fragile to handle as a ring element than would the relatively thin abrasive portion G by itself.

Iilustratively, the various ring elements including the abrasive annulus A can be made upin quantity in suitable sizes or dimensions and compounded as to ingredients to have different thergent thermal coeillcients, and each increment of stress imposed upon any one of the elements by another and hence the maximum strain which any one element is to assume can be quite definitely controlled and kept within exceedingly modest or low limits. Moreover, each junction or union assumes a stress or strain that is likewise relatively small and quite definitely controllable. In any given case the relative dimensional change per increment of temperature rise between any two successive elements can be closely controlled or limited by appropriately compounding or selecting the materials to have a corresponding differential between their respective thermal coeflicients. As a result, also, cements or compounds to form junctions may be employed that would otherwise not be usable. For example, they may have a high modulus of elasticity, a factor otherwise disadvantageous but not objecmal coefficients of expansion as above described and stored or kept on hand for use. From them, in making up cup-wheels of various depths or axial dimension, we select ring elementsof the desired succession of different thermal coemcients selected or suited to the overall difierential in thermal coefficients between the material of the abrasive annulus A and the particular or selected material of the back B, and of the desired thicknesses or axial dimensions to give the ultimate cup-wheel depth; these are assembled or stacked one-uponthe other and joined together, in the manner above described, and thus from variouslydimensioned ring elements of appropriately different thermal coeflicients, a wide range of diameters and depths of cup-wheel constructions may be readily and inexpensively produced.

To illustrate, we have shown in Fig. 2 a cupwheel of only a fraction of the axial dimension or to give the desired lesser depth of cup-wheel.

' Particularly when interposed ring elements become of relatively small thickness, as suggested in Fig. 2, is it desirable to employ in the abrasive annulus A a layer of vitrified bond material indicated at L interposed between and joining the portions U and G, for reasons above indicated and in greater detail hereinafter set forth.

As illustrative of one manner of giving the various ring elements graded or progressively different thermal coeflicients of expansion where the tionable here because relative expansitivities between the two parts thereby Joined together are definitely controllable and may be suited to the particular modulus characteristic of such cements.

Preferably the abrasive annulus A is of composite construction. particularly where diamond abrasive grains are employed and therefore have to be employed in a layer of relatively small depth, for example, on the order of or M; of an inch, and as shown in Fig. 1, the abrasive layer indicated at G is superimposed upon and Joined to an underlaycr U which maybe 0! any approring elements are ceramic or vitrified-bonded. we may use the same bond material in all of them and employ difierent body materials for each or vary progressively the proportions of body materials employed, the body materials having physical characteristics such as will appropriately alter or difierently fix the thermal expansivities of the various parts. For example, we may embody in the ring elements R R=,.R and R progressively increasing proportions of fused alumina in appropriate grit size such as from '200 to 300, in respective amounts varying from about 1% by volume for ring element R to about 12 or 15% by volume for ring element It, and with the abrasive ring element A substantially devoid of such fused alumina. In such case the thermal coefllcients of expansion of the series of superimposed ring elements are progressively greater in the direction from the abrasive'element A to the ring element B. As another illustration, vitreous silica and fused alumina, in appropriate grit sizes such as those above mentioned, may be added in admixture to the mixes of the various parts, with the proportion of fused alumina increasing progressively as above indicated and in the above-mentioned direction; in such case the abrasive ring element A may have included in its mix and hence in its structure only vitreous silica, the ring elements R to R including both vitreous silica and fused alumina in admixture, with the fused alumina progressively increasing in proportion and, if desired, the proportion of vitreous silica may be correspondingly, or substantially so, progressively diminished. As illustrative of the kinds of ceramic or vitrifiable bond materials that may be employed, the bond may be compounded according to the following:

Such a bond material, in powdered form, may be made into an abrasive mix for the abrasive annulus A by adding thereto, in suitable proportion,

the diamond abrasive grain in the desired grit size, with or without body material such as comm-inuted vitreous silica. Insimilar manner the ring elements R to R may be made up, but instead of using diamond abrasive grain therein, any suitable granular body material may be employed, including body materials as above described for progressively grading the thermal coefficients of expansion of the series of parts employed. These various mixes may be made up in known manner by utilizing any suitable temporary liquid form of binder to plasticize the mixes and to give the respective ring elements, after molding under suitable pressure, appropriate strength in the green stage. Protective agents such as aluminum, silicon, or boron, or compounds of silicon or compounds of boron, may be employed in these mixes and, if desired, may also serve as body materials according to the teachings of Patents Nos. 2,356,937 and 2,356,938. In the foregoing we have set forth several ways in which the thermal expansivities of the various parts may be varied or predetermined in relation to each other; as illustrative of other ways of predetermining the thermal coefficients of expansion, mixed glasses may be employed in the bond according to the teachings of Patent No. 2,343,218, and by the controls and means described in that patent, the desired thermal coefficient of expansion for each of the graded or progressively difiering parts of the structure of our invention may be achieved. Furthermore, and as earlier above indicated, the bonded structures employable in our invention can also be of the porcelanic as well as the glassy type.

As earlier pointed out, the various junctions J, J .1, etc., may be made in various ways and of various materials, including vitrifiable or ce- 8 ramic bond materialsinterposed in suitable layers during assembly and prior to firing; also, as pointed out above, the diamond-containing abrasive portion G and the vitrified-bonded portion U may be joined together by a layer L of raw vitrifiable bond material interposed .between the two during molding and prior to firing. Thereby and in accordance with certain other features of our invention, we are enabled to achieve certain further structural and functional coactions and advantages in the use of such vitrified bond layers for the junctions, particularly also where the part B is also of vitrified-bonded structure, in which case also the junction J comprises a layer of vitrified bond material. Where precision grinding operations on hard materials are necessary, it is particularly desirable that the diamond abrasive portion be relatively rigid and unyielding for achieving maximum accuracy and trueness of the work being ground; as earlier noted, the abrasive annulus A can be given the desired hardness and the desired high modulus of elasticity. The relatively thin layer L of vitrified bond material interposed between the parts G and U forms not only a hard and resistant direct under-support for the abrasive portion G, but also stiffens and reinforces not only the abrasive annulus G but also the under-layer U and conditions them individually and conjointly to better resist the effects of the pressures exerted during grinding and frequently concentrated substantially along a line of the abrasive surface, thus resisting strain even under concentrated grinding stresses. The above-described property of lack of ductility or plasticity when subjected to concentrated stress gives the junction-forming layer L, when made of vitrified bond material, the capacity of taking from the thin abrasive portion G, and in effect distributing within itself and throughout a larger area, localized stresses caused by concentrated grinding pressures imposed on the grinding face of the grinding annulus G, and thus contributes to the capacity of the thin abrasive portion G to resist strain or deformation. succeeding junctions J, J etc., also made of vitrified bond material, and particularly where the parts R R etc., are of small thickness, coact in a generally similar way,

whether one or more is employed, to further dis-- tribute, and successively from one lamina to the next, such transmitted and distributed stresses and thereby stiffen the structure and materially enhance its resistance to undergoing strain under grinding pressures; these actions are greatly enhanced as the thickness of the. part U and of the one or more parts R R etc., that-are employed, is diminished. By the resultant laminated structure we are enabled to provide a diamond grinding wheel that has an overall effective hardness or stiffness appropriate to function efliciently to meet the requirements of high precision grinding, without having to cope with the various and frequently difficult problems met with were it to be attempted to arrive at comparable effective hardness by making up the wheel body out of a single mix of vitrifiable material. Where substantial dimensional changes due to temperature changes are met with, we can safeguard the stiffening layers of vitrified bond material against impairment of their stiffening and backing-up actions by giving the parts progressive or successively different thermal coefficients of expan sion as above described and thus lessen the stresses and strains thermally imposed upon the stiffening layer, and thus maintain them in conshown in the drawings and above described are to be understood as illustrative and are in the form of grinding wheels comprising successive and superimposed annular members inasmuch as the many features of our invention are perhaps best understood and illustrated-by the actions .and requirements met with in grinding wheel construction and operation. However, as earlier above indicated, our invention is equally applicable to other forms of abrasive structures such as wheel segments of which a suitable number are mounted upon a common movable or rotatable support, also abrasive blocks and bones, and the like. similar problems such as those solved by our invention are met with and by constructing and mounting these articles in the manner above described with respect to the illustrative grinding wheels, such problems and difliculties are dependably overcome.

In such other abrasive articles generally It will thus be seenthat there has been provided 1 in this invention an abrasive article and a method of producing the same in which the various objects above noted, together with many thoroughly practical advantages, are successfully achieved.

As many possible embodiments may be made of the mechanical features of the above invention and as the art herein described might be varied in various parts, all without departing from the scope of the invention, it is to be understood that all matter hereinabove set forth, or shown in the accompanying drawings, is to be interpreted as illustrative and not in a limitin sense.

1. A diamond abrasive cupwheel comprising a bonded diamond abrasive annulus-and a supporting back of different thermal coefiicients of expansion wherebyrelative dimensional changes take place therebetween in response to temperature changes, and means securing 'said abrasive annulus and said back together comprising a plurality of ring elements interposed between the back and a face of said abrasive annulus, adja cent parts having bonded junctions therebetween tosecure'them together, said ring elements having respective thermal coemcients ofexpansion that are progressively differentfrom each other within the range of difference in the thermal coemcients of said abrasive annulus and of said back whereby, in response to temperature changes, the over all relative dimensional change between the abrasive annulus and the back is accompanied by intermediate and progressive relative dimensional changes in said ring elements, thereby preventing any bonded junction from being strained to an extent commensurate with the relative dimensional changes between the abrasive annulus and the back.

2. An abrasive cup wheel comprising an abrasive annulus and a supporting back of different thermal coefficients of expansion whereby relain response to temperature changes. a p ty of ring elements interposed between said back and a face of said abrasive annulus, said ring elements having progressively different thermal coefflcients of expansion intermediate of the thermal coeflicients of said abrasive annulus and said supporting back, and means joining adjacent surfaces of adjacent parts of the assemblage together whereby no joining means is subjected to a strain commensurate with that which could be caused directly by the relative dimensional changes between said abrasive annulus and supporting back in response to temperature changes.

3. .Anabrasive cup wheel comprising a succession of axially superimposed annuli of progressively diflerent thermal coeiiicients of expansion, with that annulus which is at one end of said succession having the highest thermal coefiicient and that annulus which is at the other end of said succession having the lowest thermal coefficient, the annulus at at least one end of the succession having an annular abrasive face, and means bondedly joining together adjacent faces of successive annuli, whereby, upon rise in temperature of the abrasive wheel, said progressively different thermal coeflicients of expansion of said annular means interposed between and presenting faces of substantial area to the respective faces of said annulus and said back and having a thermal coemcient of expansion intermediate of those of said annulus and said back, and means forming substantially rigid and unyielding Junctions between adjacent faces of said abrasive annulus and said annular means, and between adjacent faces a of thev latter and said back and each subjected,

upon rise in temperature of the parts, to a lesser strain than could be caused directly by the relative dimensional changes, betweensaid abrasive annulus and said back for the same temperature change. I

5. An abrasive cup wheel comprising an abrasive annulus and a supporting back, said annulus said abrasive annulus and said back and differing from each other substantially in succession.

6. An abrasive cup wheel comprisinga supporting' back and a plurality of superimposed rigid annular members having bond means coactin between adjacent faces thereof to secure them together and to said back, at least that annular member that is remote from the back having an abrasive surface and said rigid annular members having thermal coefllcients of expansion that in- I crease from one endmost annular member to the other.

tive dimensional changes take place therebetween 7. An abrasive cu wheel comprising a succession of axially superimposed annular vitrifiedbonded annuli of progressively different thermal coeiiicients of expansion, with that annulus which is at one end of said succession having the highest thermal coeflicient and that annulus which is at the other end of said sucession having the lowest thermal coeflicient, of which the annulus ;at at least one end of the succession presents an annular abrasive face, and means bondedly joining together adjacent faces of successive annuli, the bonding means joining together said one end annulus and the succeeding annulus comprising a stiffening layer of vitrified glassy material, whereby, upon rise in temperature of the abrasive wheel, said progressively different thermal coefiicients of expansion of saidannuli cause the latter to respond in corresponding increments of dimensional change whereby no joining bonding means is subjected to a strain commensurate with that which could be caused directly by the relative dimensional changes between the two endmost annuli.

8. An abrasive cup wheel comprising a succession of axially superimposed annular vitrifiedbonded annuli of progressively different thermal coemcients of expansion, with that annulus which is at one end of said succession having the highest thermal coefficient and that annulus which is at the other end of said succession having the lowest thermal coefiicient, of which the annulus at at least one end of the succession presents an annular abrasive face of vitrified-bonded diamond grains, and means bondedly joining together adjacent faces of successive annuli, said joining means comprising a stiffening layer of vitrifiedv glassy material interposed between and bonded to the adjacent faces of successive annuli, whereby, upon rise in temperature of the abrasive wheel, said progressively different thermal coeflicients of expansion of said annuli cause the latter to respond in corresponding increments of dimensional change whereby no stiffening layer is subjected to strain commensurate with that which could be caused directly by the relative dimensional changes between the two endmost annuli in response to temperature changes.

9. A. diamond abrasive cup wheel comprising a supporting back and a plurality of superimposed vitrified-bonded annular members of which that one remote from the supporting back comprises diamond abrasive grains, said vitrified-bonded annular members being joined together by stiffening means comprising vitrified glassy material in layer-like conformation, and means joining that annular member adjacent said back to the latter throughouttheir adjacent surfaces, the annular members between said remote abrasive member and said back having thermal coefficients of expansion intermediate of those of said rev mote annular member and of said back.

10. An abrasive wheel comprising a, supporting back and a plurality of superimposed relatively rigid annular members having bond means of relatively high modulus. of elasticity coacting between adj acentfaces thereof to secure them together. and to said back, at least that annular member that is remote from the back having an abrasive surface and said annular members having respectively thermalcoeificients of expansion that increase in the direction from one endmost annular member tothe other whereby to lessen the strains imposed upon said bond means of high modulus of elasticity in response to relative dimensional changes between adjacent annular members due to temperature changes.

11. An abrasive cup wheel comprising asuccession of axially superimposed annular vitrifiedbonded annuli of progressively different thermal coefilcients of expansion, with that annulus which is at one end of said succession having the highest thermal coefficient and that annulus which is at the other end of said succession having the lowest thermal coefiicient, of which the annulus at at least one end of thesuccession presents an annular abrasive face, and means bondedly joining together adjacent faces of successive annuli, the bonding means joining together said one end annulus and the succeeding annulus-comprising a stiffening layer of vitrified glassy material and the bonding means joining together said lastmentioned annulus and succeeding annuli comprising a bonding material of relatively high modulus of elasticity, whereby, upon rise in temperature of the abrasive wheel, said progressively succession of superimposed members the member at at least one end of the succession presents an abrasive face, and means bondedly joining together adjacent faces of successive members.

13. An abrasive article comprising a plurality of superimposed vitrified-bonded members of which at least one endmost member presents an abrasive face of vitrified-bonded abrasive grains, said vitrified-bonded elements containing fused alumina grain, and bonding means coacting between adjacent faces of said members to secure them together, the fused alumina grain content of said members being progressively greater in successive superimposed vitrified-bonded members whereby the latter have progressively difiering thermal coefllcients of expansion so that the bonding means securing any two successive members together is subjected to strain less than that which could be caused directly by the relative dimensional changes between the two endmost members in response to temperature change.

14. An abrasive cup wheel comprising a succession of axially superimposed annular vitrifiedbonded annuli of progressively different thermal coeflicients of expansion, with that annulus which is at one end of said succession having the highest thermal coefiicient and that annulus which is at the other end of said succession having the lowest thermal coefiicient, of which the annulus at at least one end of the succession presents an annu lar abrasive face, and means bondedly joining together adjacent faces of successive annuli, the bonding means joining together said one end annulus and the succeeding annulus comprising a stiffening layer of vitrified glassy material and the bonding means joiningtogether said lastmentioned annulus and succeeding annuli comprising a resinoid bonding material of relatively high modulus of elasticity, whereby, upon rise in temperature of the abrasive wheel, said progressively different thermal coefficients of expansion of said annuli cause the latter to respond in corresponding increments of dimensional change 13 whereby no joining bonding means is subjected to a strain commensurate with that which could be caused directly by the relative dimensional changes between the two endmost annuli.

15. An abrasive cup wheel comprising a succession of axially superimposed annular vitrifiedbonded annuli of progressively different thermal coeflicients of expansion, with that annulus which is at one end of said succession having the highest thermal coefiicient and that annulus which is at the other end of said succession having the lowest thermal coeflicient, of which the annulus at at least one end of the succession presents an annular abrasive face, and means bondedly joining together adjacent faces of successive annuli, the bonding means joining together said one end annulus and the succeeding annulus comprising a stiffening layer of vitrified glassy material and the bonding means joining together said last-mentioned annulus and succeeding annuli comprising a vitrified bonding material .of relatively high modulus of elasticity, whereby, upon rise in temperature of the abrasive wheel, said progressively different thermal coefficients of expansion of said annuli cause the latter to respond in corresponding increments of dimensional change whereby no joining bonding means is subjected to a strain commensurate with that which could be caused directly by the relative dimensional changes between the two endmost annuli.

16. An abrasive article comprising a succession of superimposed vitrified-bonded members compounded to have thermal coefficients of expansion of respective magnitudes that increase from one end member of the succession to the other end member and of which succession of members an endmost member comprises a layer of substantial thickness of vitrified-bonded diamond abrasive grains integrally united therewith by a relatively thin stifiening layer of vitrified glassy material to back up said layer of vitrified-bonded diamond grains, and means bondedly joining together adjacent faces of said members, said differing thermal coefficients of expansion causing said members, in response to temperature change, to partake of respective increments of dimensional change whereby strain to which any joining bonding means between adjacent members is subjected in response to temperature changes is less than that which could be caused directly by the relative dimensional changes between the two endmost members. I

17. A diamond abrasive article comprising an assemblage of a plurality of rigid vitrified-bonded parts in superimposed relation and presenting adjacent faces in juxtaposition to each other, said superimposed parts being compounded to have effective relative thermal coeflicients of expansion that substantially increase from one end to the other of said plurality of superimposed Parts, an endmost part comprising a hard vitrified bonded diamond-grain-containing layer of relatively small thickness and providing said assemblage of superimposed parts with an external diamond abrasive face, said parts being integrally united by bonding means coacting with adjacent faces of said parts to secure them together and the bonding means between said diamond-grain-containing layer and its adjacent vitrified-bonded part comprising a stiffening and uniting relatively thin layer of vitrified glassy material bonded to each and forming a deformation-resisting backing for said diamond-grain-containing layer, said relatively differing thermal coefficients of expansion causing said parts, in respouse to temperature change, to partake of res'pective dimensional changes thereby materially to relieve the junctions between said parts from deleterious strain in response to changes in temperature.

18. A diamond abrasive cup wheel comprising a supporting back, an abrasive annulus comprising diamond abrasive grains bonded by a vitrified bond material that is hard and compressionresisting, said abrasive annulus being relatively thin in a direction transverse to its grinding face and having the diamond grains substantially uniformly distributed throughout it, a plurality of vitrified-bonded annular members interposed between said abrasive annulus and said back, said vitrified-bonded members and said vitrifiedbonded diamond abrasive annulus being joined together at their respective adjacent faces by respectively interposed relatively thin stiffening layers of hard compression-resisting vitrified glassy material of relatively high modulus of elasticity and thereby successively back up said vitrified annular members and said vitrified-blinded diamond abrasive annulus against strains which tend to be caused by concentrated stresses due to localized grinding pressures at said grinding face, whereby said stresses are progressively distributed throughout the laminated structure comprising said plurality of vitrified-bonded annular members and said stiffening layers, and means joining that annular member adjacent said back to the latter through their adjacent surfaces.

19. A diamond grinding wheel for precision grinding and adapted to withstand substantial localized grinding pressures, said wheel comprising an abrasive annulus of vitrified-bonded diamond grains, said annulus being of relatively small thickness in a direction transverse to its grinding face and having the diamond grain sub-. stantially uniformly distributed throughout it,

and supportingv and stress-distributing means therefor comprising a rigid compression-resisting vitrified-bonded annulus underlying said abrasive annulus and integrally united therewith by a stiffening layer of hard compression-resisting vitmoted.

, LOWELL H. MILLIGAN. ROBERT H. LOMBARD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Date Number Name 687,962 Hyde Dec. 3, 1901 1,336,?51 Linbarger Apr. 13, 1920 2,065,942 Lane Dec. 29, 1936 2,102,343 Whitcomb et a1. Dec. 14, 1937 2,121,746 Sanford June 21, 1938 Hinnuber et al. Apr. 7, 1942 

